Various aircraft manufacturers have been and continue to be interested in producing supersonic aircraft for general civil use in the United States and elsewhere. Current regulations, however, prohibit at least supersonic flight over the continental United States and many other countries due to the presently objectionable nature of sonic booms. Therefore, before supersonic civil aviation becomes permissible, the sonic characteristics (i.e., the acoustic signatures) of aircraft must be modified and demonstrated to be generally acceptable.
In this regard, it is known that the N-shaped acoustic signature of a supersonic aircraft can be shaped or modified by various means to quiet the boom or otherwise render it less objectionable. See, for example, PCT Publication Number WO 03/064254 (PCT Application Number PCT/US03/0263 1) published Aug. 7, 2003, the contents of which are incorporated by reference.
Furthermore, generating a statistically meaningful database of sonic boom information in order to demonstrate the tolerable nature of modified acoustic signatures is considered to be an important step in establishing new, rational rules for acceptable supersonic flight over land. In order to generate such a statistically meaningful database, actual flight test data is required.
Ideally, flight test data would be generated using a full-scale test vehicle that duplicates exactly the aerodynamics of a future supersonic aircraft. The cost of designing, building, and operating a vehicle, and hence the cost of obtaining such flight test data, however, increases significantly with the size of the vehicle; therefore, it is desirable to generate flight test data using sub-scale aircraft. However, if a given sub-scale aircraft is simply flown at the same design point (altitude and speed) as the aircraft of which it is a model, the acoustic signature of the sub-scale aircraft will not match that of the design (full-scale) aircraft, and the data generated using the sub-scale aircraft will be of little value. Therefore, certain accommodations have to be made in order for the acoustic signature of the sub-scale aircraft to match the acoustic characteristics of the larger vehicle.
Sonic Boom Shaping
In general, a normal sonic boom is characterized by a pressure disturbance or “signature” at the ground which has an initial abrupt pressure rise (the bow shock), a gradual expansion to below ambient pressure, and then a second abrupt pressure rise (the tail shock) back to ambient. It is the two abrupt pressure rises—the bow shock and the tail shock—which an observer on the ground perceives as the classical two-pulse sonic boom: “boom-boom”.
When pressure deviation from ambient (ΔP) is plotted as a function of either distance or time, the plot has the shape of a capital “N.” The magnitude of the pressure rise is a function of the vehicle weight, its length, altitude, and speed of flight (Mach number). The duration of the boom (ΔT, i.e., the time interval between the bow and tail shocks) is primarily a function of the vehicle length, flight speed, and altitude, although vehicle shaping also has an effect on boom characteristics.
A typical N-wave sonic boom is plotted in FIG. 1, which illustrates three parameters associated with a given N-wave: the initial bow shock pressure rise (ΔPbow), the boom duration (ΔT), and the final tail shock pressure rise back to ambient (ΔPtail). In general, the pressure signature can be plotted as a function of either time or distance, and in qualitative terms the plots will appear similar. In many cases, it does not matter which independent variable is used. However, when evaluating and comparing the acoustic characteristics of sonic signatures, it is important that they be compared as functions of time, since it is pressure fluctuations over time that is perceived by the human ear.
As exemplified by FIG. 1, a typical N-wave has a duration ΔT of one to two tenths of a second, although longer or shorter booms can also occur. If the time ΔT between the bow and tail shocks is less than about 50 milliseconds ( 1/20 of a second), the human ear can not distinguish the two distinct pressure rises and the boom will be heard as a single sound. If, on the other hand, ΔT is longer than about 50 milliseconds, the boom will be heard as two sharp and distinct sounds (although atmospheric turbulence, ground and building reflections, and other factors can vary that perception).
FIGS. 2 and 3 are examples of typical sonic booms measured from overflights of actual aircraft. FIG. 2 is the boom signature from an F-15 fighter and FIG. 3 is the boom signature from a much larger aircraft, namely, the 1960's XB-70 experimental supersonic bomber. Although both booms show minor deviations from the “clean” N-wave shape of FIG. 1, both are still clearly classic N-waves, with abrupt bow shocks to peak overpressure, linear expansion, and second abrupt tail shocks back to ambient.
As noted above, it is known that the N-shaped acoustic signature of a supersonic aircraft can be shaped or modified by various means in order to reduce the noise level and annoyance of sonic booms created by the aircraft. In this regard, it is possible to make two changes to the typical signature. First, the magnitude of the overpressure ΔP can be reduced, as the noise level is in part directly related to the overpressure level. Second, the abruptness of the pressure rise can be reduced.
One way to reduce the abruptness of the pressure rise is to “replace” the single strong shock with a series of weak shocks with discrete time intervals between them, as is known in the art. A typical shaped boom signature of this type is plotted in FIG. 4, where it can be seen that the interval ΔTrise over which the pressure rises to peak overpressure ΔPpeak is a much longer period of time, e.g., as much as a third of the total signature duration. Even with the same peak overpressure ΔPpeak, a shaped boom of this type will have a very different sound characteristic than a classic N-wave boom will have. In particular, while the abrupt pressure rise of a classic N-wave boom will be heard as a very sharp crack that startles the observer and is responsible for much of the objectionable aspects of the boom, a shaped signature with extended rise time, e.g., as shown in FIG. 4, will be heard as more of a rumble (like distant thunder) than a sharp crack. Thus, although the observer will still hear a shaped boom, it will not be as startling and objectionable as a classic N-wave boom.
It should be appreciated that shaping of the tail shock is also an important aspect that can be manipulated in order to eliminate the objectionable aspects of sonic boom, although it has not received as much attention and research as has shaping the bow shock.
Operating Point Adjustment for Acoustic Matching
As noted above, if a given sub-scale aircraft is simply flown at the same design point (altitude and speed) as the aircraft of which it is a model, the acoustic signature of the sub-scale aircraft will not match that of the full-scale aircraft, and the data generated using the sub-scale aircraft will be of little value. More particularly, in that case, a sub-scale demonstrator will produce a boom signature with a shorter duration and weaker shocks than the full-scale aircraft's boom signature under like conditions.
This is illustrated in FIG. 5, where the ground signatures of full-scale and 75% scale low-boom vehicles, flown at the same flight condition, are compared. (In this and all subsequent examples, the boom signatures and the values of their various associated metrics or parameters have been generated computationally based on an equivalent area distribution representing a low boom supersonic business jet.) Focusing on the bow shock, the example shows that by scaling down to 75%, the rise time ΔTrise to peak overpressure ΔPpeak is significantly reduced as compared to the full-scale vehicle; that is, from 56 milliseconds to 36 milliseconds. The magnitude of the overpressure level is also significantly reduced. Similar changes have also taken place at the tail shock.
The classical way to compensate for the reduced overpressure level generated by the sub-scale vehicle is to fly the sub-scale vehicle at a reduced altitude, since operating at a lower altitude will cause the overpressure level to be increased as illustrated in FIG. 6. (The shorter propagation distance from the reduced cruise altitude to the ground results in less attenuation of the pressure disturbance, thus increasing the overpressure level that reaches the ground.) For example, in FIG. 6, the ground signature produced by a given vehicle at an altitude of 55,000 feet is compared to the signature produced by the same vehicle at a reduced altitude of 45,000 feet. Noticeably, the magnitude of the peak overpressure, as well as the strength of all of the small, incremental shocks which together make up the pressure rise, is increased. Furthermore, the duration of the signature is slightly increased, but not by a significant amount. Therefore, when testing sub-scale vehicles in order to simulate the acoustic characteristics of a full-scale design vehicle, it is known to operate the sub-scale vehicle at a lower altitude so that the increase in maximum overpressure ΔP attributable to operating at a lower altitude offsets or compensates for the reduction in overpressure due to scaling, and the full-scale overpressure levels are matched.
FIG. 7 is an exemplary “map” showing rise times and peak overpressures associated with an array of scale factor/altitude combinations for a given vehicle operating at a given Mach number, viz., at the design point Mach number. In this particular example, the map has been generated for an aircraft that, full-scale, weighs 120,000 pounds, has an effective length of 165 feet, and is equipped with a two-stage extendable spike on the forward fuselage, which forms the first part of the signature shaping. In this regard, U.S. Pat. No 6,698,684 describes an example of such a spike and therefore is hereby expressly incorporated herein by reference. Furthermore, the particular aircraft of this example has a full-scale design operating point of Mach 1.8 at 55,000 feet altitude. Each node or intersection in the map represents a specific combination of vehicle scale factor and operating altitude at the set value of Mach 1.8, and sonic boom signatures have been computationally determined for each node. The abscissa value of each node indicates the (shaped) rise time to peak overpressure associated with the particular scale factor/altitude combination (at Mach 1.8), and the ordinate value of each node indicates the peak overpressure value associated with the particular scale factor/altitude combination (at Mach 1.8).
The node at a scale factor of 1 operating at 55,000 feet (at Mach 1.8) represents the full-scale operating point, and the horizontal and vertical “cross-hairs” (axis intercepts) indicate the rise time and overpressure associated with it. “Moving” horizontally to the left from the design point, as illustrated in FIG. 8, will identify (e.g., by interpolation) for any particular scale factor utilized the proper altitude at which to operate a sub-scale vehicle in order to generate the same overpressure level ΔP as the full scale vehicle. The curve in FIG. 9 summarizes the ΔP match points (required sub-scale operating altitude) for each scale factor.
Deficiency of Classical Acoustic Matching
It has been observed, however, that simply matching peak overpressure does not render the acoustic signature of a sub-scale vehicle the same as the acoustic signature of the design vehicle it is desired to represent. For example, the ground acoustic signatures of sub-scale vehicles having scale factors of 75%, 50%, and 25%—each operating at Mach 1.8 and at the associated overpressure match altitude point—are compared to the ground acoustic signature of a full-scale vehicle operating at Mach 1.8 in FIG. 10. Noticeably, although all four of the peak overpressures match, the four signature shapes are clearly quite different and will exhibit significantly different sound characteristics. Obviously, none of the sub-scale vehicles flown under these matched conditions provides a satisfactory model of a full-scale aircraft of interest.
At 25% scale, the duration of the boom signature is below the 50 millisecond threshold, and the boom will generally be perceived as a single loud crack rather than a double boom.
At 50% scale, the signature duration does exceed the 50 millisecond threshold, but at 80 milliseconds, the duration is only half of the full scale vehicle's duration. Furthermore, the rise time to peak overpressure of the shaped portion of the bow shock for the 50% vehicle is reduced by a greater percentage than the scale factor and thus produces less than half of the full scale airplane's rise time—down by more than half to just 37.3% of the full-scale rise time. Because the rise time and the exact characteristics of the pressure variation from zero to peak determine much of the sound characteristics of a sonic boom, a signature with only half (or less) of the rise time of a full-scale signature will not accurately reproduce or reflect the actual sound quality of the full-scale signature.
At a scale factor that is as large as 75%, the total signature length is 120 milliseconds, which is long enough that an observer will be able to distinguish clearly the bow and tail shocks. However, that rise time is still only two-thirds of the full-scale rise time, and again the sub-scale signature will produce different sound characteristics than the full-scale signature.
FIG. 11 graphically illustrates this observation with regard to the respective scaling rates of the significant aspects of acoustic signatures obtained following the classical scaling technique explained above (simply reducing operating altitude of the sub-scale vehicle to compensate for the reduced overpressure generated by the sub-scale vehicle). More particularly, FIG. 11 plots on the vertical axis the ratio of any particular measure or parameter of the sonic boom signature (such as rise time) for the sub-scale vehicle to that same measure for the full scale vehicle. A measure that scales at the same rate as the vehicle scale factor will be represented by a 45 degree line on the plot. Measures that scale slower than the vehicle will be above the 45 degree line, and measures that scale faster than the vehicle will be below the 45 degree line.
From this plot, it is observed that the total signature length (time) scales at approximately the same rate as the vehicle. For example, the 75% vehicle has a total signature which is approximately 77% as long as the full-scale signature; the 50% vehicle has a total signature which is approximately 53% as long as the full-scale signature; and so forth. Thus, although maximum overpressure ΔP of a full-scale vehicle might be matched with a sub-scale vehicle by using classical scaling techniques, the total signature length, which bears on how a given sonic boom is perceived, will not be matched.
More significantly, FIGS. 10 and 11 illustrate that rise time to peak overpressure, which plays a greater role in how a given sonic boom is perceived than total signature length does, scales faster than the vehicle. Thus, the 75% vehicle has a rise time that is approximately 67% of the full-scale shaped signature rise time; the 50% vehicle has a rise time that is approximately 37% of full-scale; and the 25% vehicle has a rise time that is just 11% of the full-scale rise time.
Given the manner in which total signature length scales with vehicle scale, and even more so given the manner in which the time to rise to maximum overpressure varies with vehicle scale factor, the utility of operating a scaled shaped sonic boom demonstrator according to the classical approach—simply changing operating altitude of the sub-scale vehicle to produce the same maximum overpressure as the full-scale vehicle—is significantly limited.
As noted above, the cost to build and operate a vehicle scales with size; therefore, to reduce costs associated with building and testing vehicles for demonstrating improved acoustic signatures, using generally smaller vehicles is desired. As illustrated immediately above, however, even for sub-scale vehicles as large as 75% of full-scale, the accuracy and hence the scientific value of the signature reproduction is lacking to such an extent that use of even this scale factor is questionable. Therefore, if truly useful, meaningful sonic boom data is to be obtained while at the same time realizing the cost savings associated with using sub-scale aircraft to generate the data, revised scaling techniques must be used.